Coherent optical receiver device and coherent optical receiving method

ABSTRACT

In a coherent optical receiver device, the dynamic range considerably decreases in the case of selectively receiving the optical multiplexed signals by means of the wavelength of the local oscillator light, therefore, a coherent optical receiver device according to an exemplary aspect of the invention includes a coherent optical receiver receiving optical multiplexed signals in a lump in which signal light is multiplexed; a variable optical attenuator; a local oscillator connected to the coherent optical receiver; and a first controller controlling the variable optical attenuator by means of a first control signal based on an output signal of the coherent optical receiver; wherein the coherent optical receiver includes a 90-degree hybrid circuit, a photoelectric converter, and an impedance conversion amplifier, and selectively detects the signal light interfering with local oscillation light output by the local oscillator out of the optical multiplexed signals; and the variable optical attenuator is disposed in the optical path of the optical multiplexed signals in a stage preceding the photoelectric converter, inputs the optical multiplexed signals, and outputs them to the coherent optical receiver controlling the intensity of the optical multiplexed signals based on the first control signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of U.S. application Ser.No. 16/014,160 filed Jun. 21, 2018, which is a Continuation Applicationof U.S. application Ser. No. 14/700,280 filed Apr. 30, 2015, now U.S.Pat. No. 10,033,468 issued Jul. 24, 2018, which is a ContinuationApplication of U.S. application Ser. No. 13/885,266 filed May 14, 2013,now U.S. Pat. No. 9,048,956 issued Jun. 2, 2015, which is a NationalStage of International Application No. PCT/JP2011/072692, filed Sep. 26,2011, claiming priority from Japanese Patent Application No.2010-258021, filed Nov. 18, 2010, the contents of all of which areincorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to coherent optical receiver devices andcoherent optical receiving methods, in particular, to a coherent opticalreceiver device and a coherent optical receiving method which receiveoptical multiplexed signals by means of coherent detection.

BACKGROUND ART

It is required to further enlarge the capacity in a backbonetransmission system as the amount of information (traffic) in theInternet increases. A coherent optical transmission technology has drawnattention as one of technologies for high capacity. In the coherentoptical transmission technology, an AC (Alternating Current) signalcomponent is received which is amplified by mixing signal light and LO(Local Oscillator) light in a coherent optical receiver device. At thattime, the larger the optical output of the local oscillator (LO) lightbecomes, the larger amplifying operation acts on the signal light.Therefore, the receiving characteristics with the high S/N(Signal/Noise) ratio can be obtained by inputting the high-power localoscillator (LO) light compared with the signal light.

An example of such coherent optical receiver device is described inpatent literature 1. The coherent optical receiver device described inpatent literature 1 includes an attenuator, an optical coupler, areceiver, and a processor.

In the coherent optical receiving system, the intensity noise of theinput signal and the output of the beat signal between the input signaland the local oscillator (LO) light are different in the reduction ratewith attenuation of the input signal. That is to say, attenuating thepower of the input signal causes the intensity noise of the input signalto drop at a faster rate than the beat signal.

For this reason, in the coherent optical receiver device in patentliterature 1, it is said that the S/N ratio of the beat signal isimproved by attenuating an input signal before the input signal iscombined with a local oscillator (LO) signal. Furthermore, it is saidthat the attenuation of the input signal can be adjusted in response toreal-time measurements of the S/N ratio of the beat signal by providinga feedback loop between the processor and an adjustable attenuator.

Patent literature 2 discloses a coherent optical receiver which includesan intensity adjusting means for adjusting intensity of input signallight, a converting means for converting an analog signal into a digitalsignal, a storage means for storing an amplitude value, and a controlmeans for controlling the intensity adjusting means. The convertingmeans converts the analog signal obtained by the photoelectricconversion of the combined light which is obtained by combining thesignal light and the local oscillator light into the digital signal. Thestorage means stores a first amplitude value of the analog signal whichis obtained using an input signal light with no waveform distortion asan input signal light with the local oscillator light being turned off.The control means controls the intensity adjustment value of theintensity adjusting means so that a second amplitude value of the analogsignal, which is obtained using an input signal light in operation as aninput signal light with the local oscillator light being turned off,will become equal to the first amplitude value. By employing suchconfiguration, it is said that excellent reception characteristics canbe achieved even if input signal light is suffering various waveformdistortions.

Patent literature 1: Japanese Patent Application Laid-Open PublicationNo. 2001-249053 (paragraphs [0007]-[0016])

Patent literature 2: Japanese Patent Application Laid-Open PublicationNo. 2009-212994 (paragraphs [0017] and [0018])

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In the coherent optical transmission system, since the high-power localoscillator (LO) light is being input at all times unlike the IM-DD(Intensity Modulation-Direct Detection) system, the power dynamic rangeof the input signal light is limited. In other words, if the outputpower of the local oscillator (LO) light is turned up in order toimprove the minimum receiver sensitivity characteristics, the receivablemaximum input power becomes smaller. Therefore, the relation of theimprovement in the minimum receiver sensitivity and the extension of thetransmission distance to the input dynamic range becomes trade-off. Andin the related coherent optical receiver, there is a problem that thedynamic range of the optical input power becomes smaller compared withthe receiver of the IM-DD system due to the rating of the photodiode(PD) and the limitation of the amplification factor of thetransimpedance amplifier (TIA).

If the dynamic range becomes smaller, it becomes less able to absorb theinfluence caused by the loss fluctuation of an optical filter in anoptical communication system such as an ROADM (Reconfigurable OpticalAdd/Drop Multiplexer) filter or by the wavelength loss fluctuation dueto an EDFA (Erbium Doped Fiber Amplifier). For that reason, there is aproblem that it becomes difficult to design the whole opticalcommunication system and the related coherent optical receiver cannot beapplied to the current system.

On the other hand, the coherent optical transmission system has thefeature that it can receive only a signal in the wavelength channelmatched with the frequency of the local oscillator (LO) light. Anoptical FDM (Frequency Division Multiplexing) receiving system has beenconsidered in which it is performed using such feature to input directlyoptical multiplexed signals (multichannel) in the WDM (WavelengthDivision Multiplexing) system into a coherent receiver without passingthrough an optical filter and to select the intended channel signal bythe wavelength of the local oscillator (LO) light.

However, if the related coherent optical receiver is used in the opticalcommunication system in which an optical filter such as an optical DMUX(De-multiplexer) filter is not used in the same way as the optical FDMreceiving system, there is a problem that the dynamic range of theoptical input power becomes even narrower. The reason is that theaverage input power of the coherent optical receiver increases becausethe optical signals in a plurality of channels are input in a lump whichincludes an optical signal in an unwanted channel which is not used as achannel signal.

As described above, in the related coherent optical receiver, there is aproblem that the dynamic range of the optical input power decreases, inparticular, the dynamic rage remarkably decreases in the case ofselectively receiving the optical multiplexed signals by means of thewavelength of the local oscillator light.

The object of the present invention is to provide a coherent opticalreceiver device and a coherent optical receiving method which solve theproblem mentioned above that, in a related coherent optical receiver,the dynamic range considerably decreases in the case of selectivelyreceiving the optical multiplexed signals by means of the wavelength ofthe local oscillator light.

Means for Solving a Problem

A coherent optical receiver device according to an exemplary aspect ofthe invention includes a coherent optical receiver receiving opticalmultiplexed signals in a lump in which signal light is multiplexed; avariable optical attenuator; a local oscillator connected to thecoherent optical receiver; and a first controller controlling thevariable optical attenuator by means of a first control signal based onan output signal of the coherent optical receiver; wherein the coherentoptical receiver includes a 90-degree hybrid circuit, a photoelectricconverter, and an impedance conversion amplifier, and selectivelydetects the signal light interfering with local oscillation light outputby the local oscillator out of the optical multiplexed signals; and thevariable optical attenuator is disposed in the optical path of theoptical multiplexed signals in a stage preceding the photoelectricconverter, inputs the optical multiplexed signals, and outputs them tothe coherent optical receiver controlling the intensity of the opticalmultiplexed signals based on the first control signal.

A coherent optical receiving method according to an exemplary aspect ofthe invention includes the steps of: receiving optical multiplexedsignals in a lump in which signal light is multiplexed; selectivelydetecting the signal light interfering with local oscillation light outof the optical multiplexed signals, and outputting a signal afterdetection; and controlling the intensity of the optical multiplexedsignals based on the signal after detection.

Effect of the Invention

According to the coherent optical receiver device and the coherentoptical receiving method by the present invention, it is possible tosecure a sufficient dynamic range even if the optical multiplexed signalis selectively received by means of the wavelength of the localoscillation light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a coherent opticalreceiver device in accordance with the first exemplary embodiment of thepresent invention.

FIG. 2 is a flowchart showing an operation of a coherent opticalreceiver device in accordance with the first exemplary embodiment of thepresent invention.

FIG. 3 is a block diagram showing another configuration of the coherentoptical receiver device in accordance with the first exemplaryembodiment of the present invention.

FIG. 4 is a block diagram showing still another configuration of thecoherent optical receiver device in accordance with the first exemplaryembodiment of the present invention.

FIG. 5 is a block diagram showing a configuration of the coherentoptical receiver device in accordance with the second exemplaryembodiment of the present invention.

FIG. 6 is a flowchart showing an operation of the coherent opticalreceiver device in accordance with the second exemplary embodiment ofthe present invention.

FIG. 7 is a block diagram showing a configuration of the coherentoptical receiver device in accordance with the third exemplaryembodiment of the present invention.

FIG. 8 is a schematic view to illustrate the principle of the coherentoptical receiving system in accordance with the third exemplaryembodiment of the present invention.

FIG. 9 is a block diagram showing a configuration of the relatedcoherent optical receiver to illustrate device limiting factors.

FIG. 10 is a view showing the relation between the input signal powerand the local oscillation optical power in the related coherent opticalreceiver upon receiving optical multiplexed signals.

DESCRIPTION OF EMBODIMENTS

The exemplary embodiments of the present invention will be describedwith reference to drawings below.

The First Exemplary Embodiment

FIG. 1 is a block diagram showing a configuration of the coherentoptical receiver device 100 in accordance with the first exemplaryembodiment of the present invention. The coherent optical receiverdevice 100 includes a coherent optical receiver 110, a variable opticalattenuator (VOA) 120, a local oscillator (LO) 130 connected to thecoherent optical receiver 110, and a first controller 140.

The coherent optical receiver 110 includes 90-degree hybrid circuit 111,a photoelectric converter 112, and an impedance conversion amplifier113. The coherent optical receiver 110 receives optical multiplexedsignals in a lump in which signal light is multiplexed, selectivelydetects the signal light interfering with the local oscillation lightoutput by the local oscillator (LO) 130 out of the optical multiplexedsignals, and outputs the signal after detection.

The first controller 140 controls the variable optical attenuator (VOA)120 by means of a first control signal based on the output signal of thecoherent optical receiver 110. For example, the first control signal isdetermined based on the amplitude information obtained from the outputsignal of the impedance conversion amplifier 113, and the attenuationdegree of the variable optical attenuator (VOA) 120 is controlled by thefirst control signal at that time.

The variable optical attenuator (VOA) 120 is disposed in the opticalpath of the optical multiplexed signals in a stage preceding thephotoelectric converter 112, inputs the optical multiplexed signals, andoutputs them to the coherent optical receiver 110 controlling theintensity of the optical multiplexed signals based on the first controlsignal.

Next, the operation of the coherent optical receiver device 100 in thepresent exemplary embodiment will be described using FIG. 2. FIG. 2 is aflowchart showing the operation of the coherent optical receiver device100 in the present exemplary embodiment. First, the coherent opticalreceiver device 100 receives optical multiplexed signal in the variableoptical attenuator (VOA) 120 (step S11). The variable optical attenuator(VOA) 120 adjusts the intensity of the optical output of the opticalmultiplexed signal (step S12), and outputs it to the 90-degree hybridcircuit in the coherent optical receiver 110.

The 90-degree hybrid circuit 111 makes the input optical multiplexedsignal interfere with the local oscillation light output from the localoscillator (LO) 130 (step S13), and then the photoelectric converter 112performs the photoelectric conversion (step S14). At that time, thesignal light which interferes with the local oscillation light output bythe local oscillator (LO) 130 is selectively detected out of the opticalmultiplexed signals. The impedance conversion amplifier (transimpedanceamplifier: TIA) 113 amplifies the electric signal convertedphotoelectrically and outputs it (step S15).

The first controller 140 compares the output signal of the impedanceconversion amplifier (TIA) 113, for example, an AC amplitude value, witha predetermined reference value (step S16), and compared results areregarded as amplitude information. If the AC amplitude value is equal toor larger than the reference value (step S16/NO), the first controller140 controls the variable optical attenuator (VOA) 120 using the firstcontrol signal which is determined based on the amplitude information atthat time so as to increase the amount attenuated and decrease theoptical output (step S17). In contrast, if the AC amplitude value issmaller than the reference value (step S16/YES), the first controller140 controls the variable optical attenuator (VOA) 120 using the firstcontrol signal which is determined based on the amplitude information atthat time so as not to change the amount attenuated (step S18).

The variable optical attenuator 120 controls the intensity of theoptical multiplexed signal based on the first control signal (step S19)and outputs it to the coherent optical receiver 110.

By adopting such configuration, according to the coherent opticalreceiver device 100 in the present exemplary embodiment, it is possibleto control the intensity of the input optical multiplexed signals basedon the intensity of the optical signal detected selectively (channel)out of the optical multiplexed signals. Therefore, it is possible tooptimize the dynamic range with respect to each selected signal light(channel). Consequently, it becomes possible to secure a sufficientdynamic range even if the optical multiplexed signal is selectivelyreceived by means of the wavelength of the local oscillation light.

In contrast, in the related coherent optical receiver device describedin the background art, the average input optical power is merelymonitored in case of inputting the optical multiplexed signals. However,since the intended signal light is only a part of the received wholeoptical signal, it is impossible for the related coherent opticalreceiver device to detect the optical output of the intended signallight. Hence, it is impossible for the related coherent optical receiverdevice to improve the dynamic range of the input light in case ofselectively receiving the optical multiplexed signal by means of thewavelength of the local oscillation light.

Although the case has been illustrated in FIG. 1 in which the variableoptical attenuator (VOA) 120 is disposed in the optical path of theoptical multiplexed signals in a stage preceding the coherent opticalreceiver 110, it is not limited to this, it is also acceptable todispose it in other locations as long as in the optical path of theoptical multiplexed signals in a stage preceding the photoelectricconverter 112.

Here, the photoelectric converter 112 and the impedance conversionamplifier 113, which compose the coherent optical receiver 110, canadopt the differential type configuration, respectively. At that time,90-degree hybrid circuit 111 inputs the optical multiplexed signals andthe local oscillation light and makes them interfere, and outputs anormal-phase optical signal and a reversed-phase optical signal to thephotoelectric converter 112 respectively. And then, the photoelectricconverter 112 photoelectrically converts the normal-phase optical signaland outputs the normal-phase electric signal to the impedance conversionamplifier 113, and photoelectrically converts the reversed-phase opticalsignal and outputs the reversed-phase electric signal to the impedanceconversion amplifier 113. By adopting the differential configuration, itis possible to eliminate signal light other than the selected signallight, that is, an optical signal in an unwanted channel.

As mentioned above, the first controller 140 controls the variableoptical attenuator (VOA) 120 by means of the first control signal basedon the output signal from the coherent optical receiver 110. The firstcontrol signal based on the output signal of the coherent opticalreceiver 110 includes a control signal from a signal processor disposedin a stage following the coherent optical receiver 110. That is to say,the coherent optical receiver device 100 in the present exemplaryembodiment can be configured which includes an analog-to-digitalconverter 150 and further a digital signal processor (DSP) 160 in astage following the coherent optical receiver 110. At that time, asshown in FIG. 3, for example, it is possible for the first controller140 to determine a first control signal based on the amplitudeinformation obtained from the output signal of the analog-to-digitalconverter (ADC) 150 and to control the variable optical attenuator (VOA)120 by means of the first control signal at that time. As shown in FIG.4, it is also acceptable to determine a first control signal based onthe amplitude information obtained from the output signal of the digitalsignal processor (DSP) 160 and to control the variable opticalattenuator (VOA) 120 by means of the first control signal at that time.In this case, it is possible to mount the first controller 140 and thedigital signal processor (DSP) 160 on the same integrated circuitelement.

The Second Exemplary Embodiment

Next, the second exemplary embodiment of the present invention will bedescribed. FIG. 5 is a block diagram showing a configuration of thecoherent optical receiver device 200 in accordance with the secondexemplary embodiment of the present invention. The coherent opticalreceiver device 200 includes the coherent optical receiver 110, thevariable optical attenuator (VOA) 120, the local oscillator (LO) 130connected to the coherent optical receiver 110, and the first controller140.

The coherent optical receiver 110 includes the 90-degree hybrid circuit111, the photoelectric converter 112, and the impedance conversionamplifier 113. The coherent optical receiver 110 receives opticalmultiplexed signals in a lump in which signal light is multiplexed,selectively detects the signal light interfering with the localoscillation light output by the local oscillator (LO) 130 out of theoptical multiplexed signals, and outputs the signal after detection.

The first controller 140 controls the variable optical attenuator (VOA)120 by means of the first control signal based on the output signal ofthe coherent optical receiver 110. For example, the first control signalis determined based on the amplitude information obtained from theoutput signal of the impedance conversion amplifier 113, and theattenuation degree of the variable optical attenuator (VOA) 120 iscontrolled by the first control signal at that time.

The variable optical attenuator (VOA) 120 is disposed in the opticalpath of the optical multiplexed signals in a stage preceding thephotoelectric converter 112, inputs the optical multiplexed signals, andoutputs them to the coherent optical receiver 110 controlling theintensity of the optical multiplexed signals based on the first controlsignal.

The configuration above is similar to that of the coherent opticalreceiver device 100 in the first exemplary embodiment. The coherentoptical receiver device 200 in the present exemplary embodiment furtherincludes a splitter (TAP) 250 which extracts a part of the opticalmultiplexed signals output from the variable optical attenuator (VOA)120 to the 90-degree hybrid circuit 111, a photodetector 260, and asecond controller 270.

The photodetector 260 converts the optical multiplexed signals inputfrom the splitter (TAP) 250 into an electric signal and outputs theelectric signal at that time to the second controller 270 as alight-receiving signal. The second controller 270 controls theattenuation degree of the variable optical attenuator (VOA) 120 by meansof a second control signal based on the light-receiving signal obtainedfrom the photodetector (PD) 260. At that time, the variable opticalattenuator (VOA) 120 controls the intensity of the optical multiplexedsignals based on the second control signal, and outputs them to thecoherent optical receiver 110.

Therefore, according to the coherent optical receiver device 200 in thepresent exemplary embodiment, it becomes possible to control thevariable optical attenuator (VOA) 120 based on the intensity of allreceived optical multiplexed signals (all channels) in addition to theintensity of the optical signal detected selectively (channel) out ofthe optical multiplexed signals. Therefore, it is possible to preventthe excessive optical signal from inputting into the photoelectricconverter 112 in the coherent optical receiver 110.

Next, the operation of the coherent optical receiver device 200 in thepresent exemplary embodiment will be described using FIG. 6. FIG. 6 is aflowchart showing the operation of the coherent optical receiver device200 in the present exemplary embodiment. First, the coherent opticalreceiver device 200 receives optical multiplexed signal in the variableoptical attenuator (VOA) 120 (step S21). The variable optical attenuator(VOA) 120 adjusts the intensity of the optical output of the opticalmultiplexed signal (step S22), and outputs it to the splitter (TAP) 250.

The splitter (TAP) 250 extracts a part of the optical multiplexedsignals output from the variable optical attenuator (VOA) 120 to the90-degree hybrid circuit 111, and outputs it to the photodetector 260.The photodetector 260 converts the optical multiplexed signal input fromthe splitter (TAP) 250 into an electric signal and outputs it to thesecond controller 270 as a light-receiving signal.

The second controller 270 compares the light-receiving signal obtainedfrom the photodetector (PD) 260, for example, a direct current (DC)amplitude value, with a predetermined reference value (step S23). If theDC amplitude value is equal to or larger than the reference value (stepS23/NO), the second controller 270 controls the variable opticalattenuator (VOA) 120 using the second control signal which is determinedby the light-receiving signal at that time so as to increase the amountattenuated and decrease the optical output (step S24). In contrast, ifthe DC amplitude value is smaller than the reference value (stepS23/YES), the second controller 270 controls the variable opticalattenuator (VOA) 120 using the second control signal which is determinedby the light-receiving signal at that time so as not to change theamount attenuated (step S25).

The 90-degree hybrid circuit 111 makes the input optical multiplexedsignal interfere with the local oscillation light output from the localoscillator (LO) 130 (step S26), and then the photoelectric converter 112performs the photoelectric conversion (step S27). At that time, thesignal light which interferes with the local oscillation light output bythe local oscillator (LO) 130 is selectively detected out of the opticalmultiplexed signals. The impedance conversion amplifier (TIA) 113amplifies the electric signal converted photoelectrically and outputs it(step S28).

The first controller 140 compares the output signal of the impedanceconversion amplifier (TIA) 113, for example, an AC amplitude value, witha predetermined reference value (step S29), and compared results areregarded as amplitude information. If the AC amplitude value is equal toor larger than the reference value (step S29/NO), the first controller140 controls the variable optical attenuator (VOA) 120 using the firstcontrol signal which is determined based on the amplitude information atthat time so as to increase the amount attenuated and decrease theoptical output (step S30). In contrast, if the AC amplitude value issmaller than the reference value (step S29/YES), the first controller140 controls the variable optical attenuator (VOA) 120 using the firstcontrol signal which is determined based on the amplitude information atthat time so as not to change the amount attenuated (step S31).

The variable optical attenuator 120 controls the intensity of theoptical multiplexed signal based on the first control signal and thesecond control signal (step S32) and outputs it to the coherent opticalreceiver 110.

As described above, according to the coherent optical receiver device200 in the present exemplary embodiment, it is possible to secure asufficient dynamic range and to protect the photoelectric converter 112even if the optical multiplexed signal is selectively received by meansof the wavelength of the local oscillation light.

In FIG. 5, the case has been illustrated in which the variable opticalattenuator (VOA) 120 and the splitter (TAP) 250 are disposed in theoptical path of the optical multiplexed signals in a stage preceding thecoherent optical receiver 110. However, it is not limited to this, it isalso acceptable to dispose them in other locations as long as in theoptical path of the optical multiplexed signals in a stage preceding thephotoelectric converter 112.

As mentioned above, in the present exemplary embodiment, the firstcontroller 140 determines the first control signal based on theamplitude information which is obtained from the output signal of theimpedance conversion amplifier (TIA) 113, and controls the variableoptical attenuator (VOA) 120 by means of the first control signal atthat time. However, it is not limited to this; it is also acceptable touse the amplitude information obtained from the output signal of theanalog-to-digital converter 150 or the digital signal processor (DSP)160, as is the case with the first exemplary embodiment.

The Third Exemplary Embodiment

Next, the third exemplary embodiment of the present invention will bedescribed. In the present exemplary embodiment, a case will be describedas an example in which the dual polarization quadrature phase shiftkeying (DP-QPSK) system is employed as a modulation method.

FIG. 7 is a block diagram showing a configuration of a coherent opticalreceiver device 300 in accordance with the third exemplary embodiment ofthe present invention. The coherent optical receiver device 300 includesa coherent optical receiver 310, the variable optical attenuator (VOA)120, the local oscillator (LO) 130 connected to the coherent opticalreceiver 310, and a first controller 340.

The coherent optical receiver 310 includes a 90-degree hybrid circuit311, a photoelectric converter 312, and an impedance conversionamplifier 313. The coherent optical receiver 310 receives opticalmultiplexed signals in a lump in which signal light is multiplexed,selectively detects the signal light interfering with the localoscillation light output by the local oscillator (LO) 130 out of theoptical multiplexed signals, and outputs the signal after detection. Thevariable optical attenuator (VOA) 120 is disposed in the optical path ofthe optical multiplexed signal in a stage preceding the photoelectricconverter 312, inputs the optical multiplexed signal, and outputs itcontrolling the intensity of the optical multiplexed signal based on afirst control signal from the first controller 340. The configurationabove is similar to that of the coherent optical receiver device 100 inthe first exemplary embodiment.

The coherent optical receiver device 300 in the present exemplaryembodiment further includes an analog-to-digital converter (ADC) 350 anda digital signal processor (DSP) 360 in a stage following the coherentoptical receiver 310. The first controller 340 determines the firstcontrol signal based on the amplitude information obtained from theoutput signal of the digital signal processor (DSP) 360, and controlsthe variable optical attenuator (VOA) 120 by means of the first controlsignal at that time.

The configuration of the coherent optical receiver device 300 will bedescribed below more specifically. As shown in FIG. 7, phase-modulatedoptical multiplexed signals are input in a lump into the coherentoptical receiver device 310 from its signal light input port. On theother hand, local oscillation light is input from the local oscillator(LO) 130 at its local oscillation (LO) light port. The coherent opticalreceiver 310 includes a polarization beam splitter (PBS) 314 in an inputside of the signal light and a beam splitter (BS) 315 in an input sideof the local oscillation light.

The variable optical attenuator (VOA) 120 inputs the optical multiplexedsignal (multichannel), adjusts it within the range of an intendedoptical output by attenuating the intensity of the optical multiplexedsignal based on the first control signal from the first controller 340,and outputs it to the coherent optical receiver 310.

The optical multiplexed signal whose optical output is adjusted isseparated into two polarized light beams by the polarization beamsplitter (PBS) 314 composing the coherent optical receiver 310, whichare input into the 90-degree hybrid circuits (90-Hybrid) 311respectively. The optical multiplexed signal is separated into anin-phase component (I) and a quadrate-phase component (Q) in the90-degree hybrid circuit 311, each of which is fed by the differentialinput into a photodiode (PD) as the photoelectric converter 312. In moredetail, the beat output composed of the optical multiplexed signal andthe local oscillator (LO) light is branched by diversity branch withrespect to each of the polarization, the phase, and the intensity, theneach of eight kinds of optical signals in total is input into thephotodiode (PD).

Only the AC signal component is extracted by a differential amplifier asthe impedance conversion amplifier 313, and is amplified to the outputamplitude suitable for the analog-to-digital converter (ADC) 350 in thefollowing stage. After that, those signals are converted by each of fouranalog-to-digital converters (ADC) 350 into digital signals, and areprocessed as signals mapped onto two I/Q planes in the digital signalprocessor (DSP) 360.

The digital signal processor (DSP) 360 performs a process so that theoutput amplitude of the analog-to-digital converter (ADC) 350 may becomeconstant. The first controller 340 determines the first control signalby using a processing signal at that time, and controls the variableoptical attenuator (VOA) 120 by the first control signal at that time.The variable optical attenuator (VOA) 120 adjusts the opticalmultiplexed signal within the range of an intended optical output byattenuating its intensity based on the first control signal.

In this way, by means of placing the variable optical attenuator (VOA)which controls the optical power based on the output amplitude of theanalog-to-digital converter (ADC) 350, it is possible to control theoptical power input into the coherent optical receiver based on theintensity of the selected signal light. Accordingly, it is possible toenlarge the power dynamic range in which the coherent optical receiveris available for receiving.

Although the case has been illustrated in FIG. 7 in which the variableoptical attenuator (VOA) 120 is disposed in the optical path of theoptical multiplexed signals in a stage preceding the coherent opticalreceiver 310, it is not limited to this, it is also acceptable todispose it in other locations as long as in the optical path of theoptical multiplexed signals in a stage preceding the photoelectricconverter 312.

As described above, according to the coherent optical receiver device300 in the present exemplary embodiment, it is possible to control theintensity of the input optical multiplexed signals based on theintensity of the optical signal detected selectively (channel) out ofthe optical multiplexed signals. Therefore, it is possible to optimizethe dynamic range with respect to each selected signal light (channel).Consequently, it becomes possible to secure a sufficient dynamic rangeeven if the optical multiplexed signal is selectively received by meansof the wavelength of the local oscillation light.

FIG. 8 is a schematic view to illustrate the principle of the coherentoptical receiving system in the present exemplary embodiment. In thecoherent optical receiving system, the local oscillation (LO) light andthe signal light are mixed in a 90-degree hybrid circuit 401, and adifferential-type photodiode (PD) 402 receives the mixed light. In FIG.8, a configuration without polarization demultiplexing will be describedfor simplicity.

In the QPSK modulation system, the information is provided for the phaseof the signal light. It is possible to detect the light intensity andthe phase information by mixing signal light and local oscillation (LO)light with its frequency the same. At that time, one of the signal lightis mixed with the local oscillation (LO) light without change in theoptical coupler 403, and the other one is mixed with the localoscillation (LO) light which has passed through the 90-degree phasedifferential unit 404. As a result, it is possible to obtain the cosine(cos) component and the sine (sin) component of the phase informationrespectively. It is found by receiving those optical signals in thephotodiode (PD) 402 which phase each optical signal exists in at themoment. Such coherent optical receiving system has the followingfeature. That is to say, it is possible to correct the frequencyfluctuation of the signal light or the local oscillation (LO) light bymeans of the digital signal processing, and to compensate the chromaticdispersion or the polarization mode dispersion by using the phaseinformation by means of the digital signal processing.

Next, device limiting factors of the related coherent optical receiverwill be described. FIG. 9 is a block diagram showing a configuration ofthe related coherent optical receiver to illustrate device limitingfactors. As shown in the figure, the maximum input power of the actualoptical signal is determined by the maximum input limitation of aphotodiode (PD) (A), an amplification factor of an impedance conversionamplifier (TIA) (B), and a limiting condition of the signal outputamplitude (C). Only the maximum amplification factor of TIA (B) becomesa problem if the input optical power is small. However, if the inputpower is large, the maximum input limitation (maximum input rating) of aPD (A), the minimum amplification factor of a TIA (B), and the maximumvalue of the signal output amplitude (C) become problems.

As a concrete example, the input limitation of the related coherentoptical receiver in the case of receiving optical multiplexed signals(multichannel) will be described. FIG. 10 is a view showing the relationbetween the input signal power and the local oscillation optical powerin the related coherent optical receiver upon receiving opticalmultiplexed signals. The horizontal axis represents the input signalpower, the vertical axis represents the local oscillation (LO) opticalpower, and the figure illustrates a case in which signal light includesone hundred channels.

In FIG. 10, the area (I) represents the limitation due to the maximumrating of a PD, the area (II) represents the limitation due to the uppergain limit of a TIA, and the area (III) represents the limitation due tothe lower gain limit of a TIA. It is found from the figure that an areapractically usable in the optical communication system is limited to thearea without the hatching (X) in the figure. The value of the range ofthe input optical signal power ΔP becomes about 8 dB if the localoscillation (LO) optical power is equal to 13 dBm. If the optical inputdynamic range is about 8 dB, it is difficult to design the opticalcommunication system. Since the coherent optical receiver is configuredso as to increase the receiving sensitivity by the local oscillation(LO) light, it is desirable for the power of the local oscillation (LO)light to be larger. Therefore, the range of the input optical signalpower is further limited.

Here, in order to enlarge the optical input dynamic range, it can beconsidered to dispose the variable optical attenuator (VOA) in a stagepreceding the coherent optical receiver, and to attenuate the opticalinput power to the receivable optical input level. However, since theoptical input signals in a plurality of channels are intermingled in thecase of receiving the optical multiplexed signals it is impossible touse them for the feedback control of the variable optical attenuator(VOA).

In contrast, in the coherent optical receiver device 300 of the presentexemplary embodiment, the first controller is configured so as to usethe output amplitude of the analog-to-digital converter (ADC) or theamplification factor of the impedance conversion amplifier (TIA) for thefeedback control of the variable optical attenuator (VOA). As a result,since it is possible to detect the control signal based on the opticalsignal in one channel, it becomes possible to control the variableoptical attenuator (VOA) appropriately and to enlarge the dynamic range.

It is also possible to use a fiber-type variable optical attenuator(VOA) or a planar lightwave circuit (PLC) type variable opticalattenuator (VOA) as the variable optical attenuator (VOA) 120 in theabove-mentioned exemplary embodiments. In addition, an integrated-typevariable optical attenuator (VOA) is also available which is integratedwith a coherent optical receiver.

The present invention is not limited to the above-mentioned exemplaryembodiments and can be variously modified within the scope of theinvention described in the claims. It goes without saying that thesemodifications are also included in the scope of the invention.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-258021, filed on Nov. 18, 2010, thedisclosure of which is incorporated herein in its entirety by reference.

DESCRIPTION OF THE CODES

-   -   100, 200, 300 coherent optical receiver device    -   110, 310 coherent optical receiver    -   111, 311, 401 90-degree hybrid circuit    -   112, 312 photoelectric converter    -   113, 313 impedance conversion amplifier    -   120 variable optical attenuator (VOA)    -   130 local oscillator (LO)    -   140, 340 first controller    -   250 splitter (TAP)    -   260 photodetector    -   270 second controller    -   314 polarization beam splitter (PBS)    -   315 beam splitter (BS)    -   150, 350 analog-to-digital converter (ADC)    -   160, 360 digital signal processor (DSP)    -   402 photodiode (PD)    -   403 optical coupler    -   404 90-phase differential unit

The invention claimed is:
 1. An optical module, comprising: an opticallight source configured to output local oscillation light; a variableoptical attenuator configured to attenuate input multiplexed signalsincluding a plurality of optical signals; a coherent hybrid mixerconfigured to receive one of the plurality of optical signals byinterfering with the local oscillation light; a photoelectric converterconfigured to convert the received optical signal into a convertedsignal; and an electrical processor configured to process an electricalsignal, the electrical signal being downstream of the photoelectricconverter, wherein the variable optical attenuator is further configuredto attenuate the input multiplexed signals based on both the electricalsignal and optical intensity of the input multiplexed signals.
 2. Theoptical module of claim 1, further comprising: an optical splitterconfigured to obtain a part of the optical intensity of the inputmultiplexed signals; and an optical power monitor configured to monitorthe part of the optical intensity, wherein the variable opticalattenuator is further configured to attenuate the input multiplexedsignals based on amplitude of the electrical signal and a monitoringresult by the optical power monitor.
 3. The optical module of claim 1,further comprising: a polarization optical splitter configured to splitthe input multiplexed signals into first polarization signals and secondpolarization signals; and a second optical splitter configured to splitthe local oscillation light into a first split light and a second splitlight, wherein the coherent hybrid mixer is further configured toreceive one of the first polarization signals and one of the secondpolarization signals by interfering with the first split light and thesecond split light, respectively.
 4. A method of receiving inputmultiplexed signals, the input multiplexed signals comprising aplurality of optical signals and the method comprising: outputting alocal oscillation light; attenuating the input multiplexed signals;receiving one of the plurality of optical signals by interfering withthe local oscillation light; converting the received optical signal intoa converted signal; and processing an electrical signal, the electricalsignal being downstream of the converted signal, wherein the attenuatingof the input multiplexed signals includes attenuating the inputmultiplexed signals based on both the electrical signal and opticalintensity of the input multiplexed signals.
 5. The method of claim 4,further comprising: obtaining a part of the optical intensity of theinput multiplexed signals; monitoring the part of the optical intensity;and attenuating the input multiplexed signals based on amplitude of theelectrical signal and a monitoring result.
 6. The method of claim 4,further comprising: splitting the input multiplexed signals into firstpolarization signals and second polarization signals; splitting thelocal oscillation light into a first split light and a second splitlight; and receiving one of the first polarization signals and one ofthe second polarization signals by interfering with the first splitlight and the second split light, respectively.
 7. The optical module ofclaim 1, wherein the variable optical attenuator is further configuredto attenuate the input multiplexed signals based on amplifier gain forthe electrical signal.
 8. The optical module of claim 1, wherein thevariable optical attenuator is further configured to attenuate the inputmultiplexed signals based on a limit optical intensity of thephotoelectric converter.
 9. The optical module of claim 1, wherein thephotoelectric converter comprises a photo diode.
 10. The optical moduleof claim 1, wherein the variable optical attenuator is furtherconfigured to attenuate the input multiplexed signals based on a limitelectrical amplitude of the electrical signal.
 11. The optical module ofclaim 1, wherein the variable optical attenuator is further configuredto attenuate the input multiplexed signals so as to prevent the coherenthybrid mixer from receiving an excessive optical signal.
 12. The methodof claim 4, further comprising attenuating the input multiplexed signalsbased on amplifier gain for the electrical signal.
 13. The method ofclaim 4, further comprising attenuating the input multiplexed signalsbased on a limit optical intensity of a photoelectric converter.
 14. Themethod of claim 4, wherein the converting is performed using a photodiode.
 15. The method of claim 4, further comprising attenuating theinput multiplexed signals based on a limit electrical amplitude of theelectrical signal.
 16. The method of claim 4, wherein the receiving isperforming using a coherent hybrid mixer.
 17. The method of claim 4,further comprising attenuating the input multiplexed signals so as toprevent input of excessive optical signal.